1. Field of the Invention
The present invention relates to a wire bonding method for connecting a first bonding point and a second bonding point through a wire, a wire loop having a certain shape and a semiconductor device having such a wire loop incorporated therein.
2. Description of the Related Art
Conventionally, in a process of fabricating a semiconductor device, as shown in FIG. 5A or 5B, wire bonding for connecting a pad 2a or a first bonding point A of a semiconductor chip 2 attached to a lead frame 1 and a lead 1a or a second bonding point Z of the lead frame 1 through a wire 3 has been carried out. Typically, loop shapes of the wire 3 connecting the first and second bonding points A and Z include a trapezoidal shape and a triangular shape shown in FIGS. 5A and 5B, respectively, as disclosed, for example, in U.S. Pat. No. 6,036,080 or Japanese Patent Application Laid-Open Publication No. 2000-277558.
The wire loop having a trapezoidal shape shown in 5A is formed by a sequence of steps as shown in FIG. 6. First, in step (a) of FIG. 6, a capillary 4 through which the wire 3 passes is lowered and a ball 30 which has been formed at a tip end of the wire 3 is bonded to the pad 2a of the chip 2 or first bonding point A. Then, in step (b) of FIG. 6, the capillary 4 is vertically raised to a point B while the wire 3 is delivered. Thereafter, in step (c) of FIG. 6, the capillary 4 is horizontally moved to a point C in a direction opposite from the second bonding point Z.
In general, such an operation of the capillary 4 to be moved in the direction opposite from the second bonding point Z is referred to as a “reverse operation”. As a result, the portion of the wire 3 between the points A and C is formed to be inclined and the wire 3 is formed at an upper end of the inclined portion thereof with a kink 3a by a lower end of the capillary 4. The portion of the wire 3 between the points A and C thus delivered corresponds to the height of a neck portion H (or a portion of the wire 3 between the pad 2a and the kink 3a) and will constitute the neck portion H shown in FIG. 5A.
Subsequently, in step (d) of FIG. 6, the capillary 4 is vertically raised to a point D while the wire 3 is delivered. Then, in step (e) of FIG. 6, the reverse operation of the capillary 4 is performed again, i.e. the capillary 4 is horizontally moved to a point E in the direction opposite from the second bonding point Z. As the result of this reverse operation, the wire 3 has another inclined portion extending between the points C and E, and a kink 3b is formed in an upper end of this inclined portion of the wire 3.
This inclined portion of the wire 3 thus delivered will constitute an upper base portion L (or a portion of the wire 3 between the kinks 3a and 3b) of the wire loop having a trapezoidal shape shown in FIG. 5A. Thereafter, in step (f) of FIG. 6, the capillary 4 is vertically raised to a point F so that the wire 3 is delivered by a length corresponding to a long inclined portion S (or a portion of the wire 3 between the kink 3b and the lead 1a) of the wire loop shown in FIG. 5A. Subsequently, the capillary 4 is lowered to the second bonding point Z via positions f1 and f2, so that the wire 3 is bonded to the second bonding point Z or the lead 1a. 
The wire loop having a triangular shape shown in 5B is formed by a sequence of steps as shown in FIG. 7. Since the wire loop having a triangular shape is not provided with an upper base portion (L) unlike the wire loop having a trapezoidal shape described above, in forming the wire loop of a triangular shape, the second reverse operation in steps (d) and (e) of FIG. 6 is not conducted. Therefore, in this instance, a step that corresponds to the steps (d), (e) and (f except for f1 and f2) of FIG. 6 is carried out only in step (d) of FIG. 7. More particularly, steps (a), (b) and (c) of FIG. 7 are the same as the steps (a), (b) and (c) of FIG. 6, and after the first reverse operation in step (c) of FIG. 7, the capillary 4 is vertically raised to a point F in step (d) of FIG. 7 while the wire 3 is delivered. Subsequently, in step (e) of FIG. 7, the capillary 4 is moved via positions e1 and e2 in a manner similar to that in step (f) of FIG. 6, with the result that the wire 3 is bonded to the second bonding point Z or the lead 1a. 
When a wire loop is formed by such a bonding method, a wire connection at the second bonding point Z or on the lead 1a has a crescent shape as the result of crushing with an end portion of the capillary, so that there is a possibility that the wire connection cannot maintain an adequate pull strength.
For this reason, a bonding method called security bonding has been proposed which performs ball bonding with respect to the top of the end-bonded wire. As described in Japanese Patent Application Laid-Open Publication No. 57-12530, this method comprises bonding a wire to a second bonding point, then cutting the wire and forming a ball at the tip of the wire and finally bonding the ball onto the second bonding point again from above. After the ball bonding, the wire is cut as is or end bonding is performed at a point which does not overlap the ball bonded position.
In the aforementioned security bonding method, however, after wire bonding with respect to the second bonding point, the wire is cut and then a ball is newly formed at the tip of the wire, followed by ball bonding with respect to the second bonding point, resulting in the method being time-consuming.